OXYGEN SOON EXHAUSTED. 605 



amount varies to almost nothing. Also, in consequence of atmospheric 

 pressure, oxygen may be forced into the sea of ground water to some 

 extent. The favorable conditions for abundant oxygen in the water when 

 it joins the belt of cementation are a very porous belt of weathering and 

 absence of luxuriant vegetation. Under such conditions the atmosphere 

 circulates somewhat freely through the' belt of weathering, and as oxygen 

 is exhausted in the belt by the processes of oxidation it is resupplied to the 

 solutions by the atmosphere. 



It is seen (pp. 609-610) that the sources of carbon dioxide are abun- 

 dant, and therefore that the process of carbonation is almost universal in 

 the belt of cementation. The situation is very different with oxygen. 

 Practically the only oxj^gen supplied is that which is carried down by 

 solutions from the belt of weathering and that which makes its way into the 

 water of the belt of cementation at the level of ground water in conse- 

 quence of atmospheric pressure. It naturally follows that, as the waters 

 move downward and the process of oxidation continues, the amount of 

 oxygen available becomes less and less. Commonly the oxygen is prac- 

 tically exhausted at very moderate depths. Therefore, while oxidation 

 takes place within the belt of cementation wherever oxygen is available, 

 for much of the belt there is no oxygen available for this process and hence 

 oxidation does not occur. This general reasoning is plainly confirmed by 

 observation. The waters of the upper part of the belt of cementation are 

 commonly oxidizing, while the deeper waters of the belt rarely contain 

 oxygen. According with this general observation, Lepsius has made exact 

 experimental tests of the amount of oxygen in waters derived from bore 

 holes, and he finds that with increase of depth there is a gradual and some- 

 what uniform decrease of oxygen contained in the ground waters." 1 Bischof 

 many years ago verified the depletion of oxygen as the result of oxidation 

 by determinations of the relative amounts of oxygen and nitrogen in springs. 

 He found in cold springs that the oxygen is not so great in proportion to 

 the nitrogen present as in the atmosphere, and in warm springs this defi- 

 ciency is even more marked, and he correctly explains this deficiency as 

 due to the "partial combination of this gas with oxidizable substances.'"' 



a Berichte Deutsch. chem. Gesell., vol. 18, 1885, p. 2487. 



6 Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drum- 

 mond, Harrison & Sons, London, 1854, vol. 1, p. 234. 



